In next generation multimedia radio communication systems which have been actively studied in recent years, It is required that a variety of information (e.g., video and radio data) beyond the early voice-oriented services is processed at a higher data transfer rate.
Accordingly, orthogonal frequency division multiplexing (OFDM) capable of having a high data transfer rate has attracted attention. The OFDM is a multi-carrier modulation scheme in which data transmission is achieved by dividing a frequency band into a plurality of orthogonal subcarriers. Orthogonal frequency division multiple access (OFDMA) provides multi-user multiplexing by combining the OFDM with frequency division multiple access (FDMA), time division multiple access (TDMA) or code division multiple access (CDMA).
A radio communication system includes a base station (BS) and at least one user equipment (UE). The UE may be fixed or mobile and can also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device, and the like.
The base station commonly refers to a fixed station which communicates with the user equipment, and it can also be referred to as a Node-B, a base transceiver system (BTS), an access point (AP), and the like. Hereinafter, an uplink (UL) refers to a transmission from the user equipment to the base station, and a downlink (DL) refers to a transmission from the base station to the user equipment.
Meanwhile, as the use of privately owned base stations or private-purpose base stations increases in addition to the use of existing service provider-owned base stations or public-purpose base stations, femtocell base stations have been used.
A femtocell is a service area of an ultra-small communication base station (a femtocell base station) used indoors, for example, in private homes or offices. The femtocell has advantages in that it can provide a fixed mobile convergence (FMC) service at a low cost by connecting the mobile terminal to the Internet, and can efficiently provide a variety of FMC services by expanding an indoor coverage and improving a call quality.
The demand for femtocell standardization was proposed as a standardization item in the 3rd Generation Partnership Project (3GPP) in the early 2007, and femtocell standardization activities have been carried out as a main issue in the 3GPP2 since June 2007. The femtocell is referred to as a home node B (HNB) in the 3GPP. TSG-RAN WG4 (Technical Specification Group-Radio Access Network Working Group 4) has led the discussion about the standardizations of 3G HNB based on wideband CDMA (WCDMA) and Long Term Evolution (LTE) Home evolved Node B (HeNB) based on LTE.
Also, in the 3GPP2, methods for minimizing the influence of the existing networks and the interfaces between macrocells have become a main issue. While various problems such as a network architecture for circuit switched/packet switched (CS/PS) service, an interface management, a handover scheme, an access system selection, a synchronization, and the like, have been under discussion, the 3GPP2 having first carried out the standardization activities took the leading position in the femtocell standardization over the 3GPP.
The femtocell can provide a high-capacity service, which has been provided in an existing wired broadband service, at a low cost under radio environments by configuring cells in a small size and remarkably increasing a frequency reuse rate, to thereby provide a high-speed data service regardless of places of users. Hence, such a femtocell technology has showed a possibility as the foundation for introduction of new services and expansion of next generation mobile communication markets following 3G.
However, since such femtocell base stations are installed in an uncoordinated, random and dense fashion, interference may occur more than in the existing macrocell base stations.
The interference between the femtocell base stations may degrade the quality of service (QoS) provided to the mobile terminal and also cause a call drop. Therefore, there is a need for technologies which can minimize interference occurring between the femtocell base stations in order for the femtocell base stations to successfully operate.
An Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) architecture in which no femtocell base stations are included will be first described. The base stations (Evolved Node-Bs (eNBs)) are connected through interfaces to Evolved Packet Cores (EPCs) which are upper nodes. The EPCs may include a Mobility Management Entity (MME) and/or a Serving-GateWay (S-GW).
In the E-UTRAN architecture, an interface between the eNBs is referred to as an X2 interface, and an interface between the EPC and the eNB is referred to as an S1 interface. That is, the X2 interface refers to an interface which directly connects the eNBs to each other, and the S1 interface refers to an interface which indirectly connects the eNBs to each other passing through the MME and/or the S-GW.
In general, interference between the macrocell base stations can be reduced by exchanging an interference control message between the macrocell base stations. In this case, the interference control message is exchanged through the X2 interface.
However, the interference control message having been used in the existing X2 interface cannot be exchanged in a femtocell environment where no X2 interface exists, and the interference-related messages associated with the S1 interface have to be newly defined.